1. Field of the Invention
The present invention relates to a method and apparatus for polishing a workpiece, and more particularly to a method and apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Such a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. An abrasive cloth is attached to the upper surface of the turntable. A workpiece such as a semiconductor wafer to be polished is placed on the abrasive cloth and clamped between the top ring and the turntable. During a polishing operation, the top ring exerts a constant pressure on the turntable, and an abrasive slurry is supplied from a nozzle over the abrasive cloth. The abrasive slurry is interposed between the abrasive cloth and the semiconductor wafer. The lower (front) surface of the semiconductor wafer held against the abrasive cloth is therefore polished while the top ring and the turntable are rotating.
In the conventional polishing apparatus, the top ring has a wafer holding surface, which is flat, for holding the semiconductor wafer at a lower surface thereof. In this polishing apparatus, a polishing rate is influenced by the relative velocity of the abrasive cloth and the semiconductor wafer, a pressing force applied to the semiconductor wafer, the amount of the abrasive slurry on the abrasive cloth, and working time of the abrasive cloth. That is, a uniform polished surface is obtainable by equalizing the above factors over the entire surface of the semiconductor wafer to be polished. Of the above factors which affect the polishing rate, the relative velocity of the surface of the semiconductor wafer to be polished and the abrasive cloth can theoretically equalize over the entire surface of the semiconductor wafer by rotating the turntable and the top ring at the same rotational speed and in the same direction.
Further, as means for making uniform a pressing force over the entire surface of the semiconductor wafer, the top ring made of hard material such as ceramics is known. Further, the polishing apparatus disclosed in Japanese laid-open patent publication No. 6-91522 has a top ring on which a diaphragm is provided to uniformize a pressing force over the entire surface of the semiconductor wafer by applying a fluid pressure to the diaphragm. The polishing apparatus disclosed in U.S. Pat. No. 4,373,991 has a top ring which has passages at the lower surface thereof to supply a fluid pressure therethrough to the semiconductor wafer.
However, in the conventional polishing methods and apparatuses, since a liquid-like abrasive slurry is supplied onto the abrasive cloth on the rotating turntable, the abrasive slurry tends to move radially outwardly by a centrifugal force. Therefore, it is difficult to uniformize the amount of the abrasive slurry over the entire surface of the abrasive cloth. In addition to nonuniformity of the amount of the abrasive slurry on the abrasive cloth, the polished surface of the semiconductor wafer is affected by the sizes of abrasive grains in the abrasive slurry and the property of solution which dilutes the abrasive grains. The polished surface of the semiconductor wafer has the tendency of representing causes of nonuniformity of the polished surface by itself.
In case of using an abrasive slurry comprising abrasive grains containing silica such as SiO.sub.2 in an alkaline solution, the polishing action is performed in such a manner that the surface of the semiconductor wafer to be polished contacts the alkaline solution and the surface etched with the alkaline solution is ground off by the abrasive grains. In this case, the surface of the semiconductor wafer tends to be over-polished at the outer peripheral portion thereof.
In case of using an abrasive slurry comprising abrasive grains containing cerium such as CeO.sub.2 in an aqueous solution, the polishing action is performed only by mechanical polishing because diameters of the abrasive grains containing cerium are larger than those of the abrasive grains containing silica and the aqueous solution does not have an etching action. In this case, the surface of the semiconductor wafer tends to be over-polished at the central portion thereof. The above phenomena are not desirable in the polishing apparatus which is used for polishing the semiconductor wafer to a flat mirror finish.